Metal alloy interconnections for integrated circuits

ABSTRACT

Novel metal-alloy interconnections for integrated circuits. The metal-alloy interconnections of the present invention comprise a substantial portion of either copper or silver alloyed with a small amount of an additive having a low residual resistivity and solid solubility in either silver or copper such that the resultant electrical resistivity is less than 3 μΩ-cm.

This is a continuation of application Ser. No. 08/071,451, filed Jun. 3,1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of integrated circuitmanufacturing, and more specifically, to interconnection metals forintegrated circuits.

2. Description of Relevant Art

Modern integrated circuits are made up of literally millions of activedevices such as transistors and capacitors formed in a semiconductorsubstrate. These devices are initially isolated from one another but arelater interconnected together to form functional circuits. The qualityof the interconnection metal drastically effects the performance andreliability of the fabricated integrated circuit. Interconnection metalsare increasingly determining the limits in performance, density andreliability of modern ultra large scale (ULSI) circuits.

FIG. 1 is a cross-sectional illustration of an interconnect structurewhich is presently widely used in the semiconductor industry. Itcomprises a silicon substrate 101 in which active devices (not shown)are formed. An interlayer dielectric (ILD) 103 is formed over thesubstrate to isolate aluminum interconnection lines 105 and 107 fromactive devices formed below. The aluminum interconnection linesinterconnect various devices to form functional circuits. The aluminuminterconnection lines are typically coupled to the substrate by metalplugs 109 and 111.

Aluminum and its alloys have been widely used as interconnection lines105 and 107 in interconnect structures because they have goodresistivity (˜3.0 μΩ-cm) and they have good adhesion to SiO₂, which istypically used as an ILD. Additionally, aluminum doped with a smallamount of copper does not diffuse through ILD 103 and interact with thesubstrate below. Unfortunately, aluminum offers poor resistance toelectromigration which increases the potential for open circuits fromvoids or short circuits from hillocks. Additionally, aluminum thin filmssuffer from stress migration which can cause voids and hillocks atrelatively low temperatures. Hillocks can cause interlevel andintralevel shorts in multilevel integrated circuits. Still anotherproblem with aluminum alloy lines is that they are susceptible tohumidity-induced corrosion.

In an attempt to improve the performance, reliability, and density ofinterconnections, alternative interconnection metals to aluminum andaluminum alloys have been proposed. Pure copper has been proposed as asubstitute for aluminum metalization. Pure copper has an extremely lowresistivity (˜1.7 μΩ-cm). A low resistivity interconnection metalimproves performance of an integrated circuit by increasing its speed.Additionally, pure copper is resistant to electromigration which makesit a much more reliable interconnection metal than aluminum and aluminumalloys.

Unfortunately, pure copper interconnections have several shortcomingswhich make them ill-suited for use in high performance reliableintegrated circuits. First, pure copper readily oxidizes whenever oxygenis present. Oxidation of copper interconnections increases theelectrical resistivity of the interconnection, thereby decreasing theperformance of the fabricated circuit. It is to be appreciated thatinterconnections can be exposed to oxygen during a number of steps inthe integrated circuit manufacturing process. For example,interconnections are exposed to oxygen in air whenever wafers sit idlebetween process modules. Interconnections can also be exposed to oxygenduring the formation of oxide based (SiO₂) interlayer dielectrics.Oxidation of copper is especially troublesome because the reactionbetween copper and oxygen is not self limiting, unlike other metals suchas aluminum, and therefore the entire interconnection can becomeoxidized which drastically increases the resistance of theinterconnection. Another problem with pure copper interconnections isthat they easily corrode (in addition to oxidizing, a form of corrosion)and cause reliability problems. Still another problem with copperinterconnections is that they readily diffuse through SiO₂ and other ILDmaterials such as polyimides. Pure copper interconnections, therefore,require a barrier layer to prevent diffusion. Barrier layers add expenseand process complexity to the fabrication of an integrated circuit.

In a similar manner, pure silver has been proposed as a substitute foraluminum alloy interconnections. Silver has an extremely low resistivity(˜1.61 μΩ-cm). Unfortunately, however, like pure copper, pure silverreadily oxidizes and corrodes creating reliability problems.

Thus, what is desired is an interconnection metal which has a lowresistivity but at the same time is resistant to void and hillockformation, oxidation, and corrosion.

SUMMARY OF THE INVENTION

Novel, high reliability, high performance copper-alloy interconnectionsfor integrated circuits are described. The copper-alloy interconnectionsof the present invention comprises copper and less than five atomicpercent of an additive having a low residual resistivity and solidsolubility in copper. The preferred composition of the copper-alloyinterconnections of the present invention comprises substantially allcopper and between 0.01-1.0 atomic percent of either palladium, niobiumor vanadium. The copper-alloy interconnections can be formed bysputtering with a single copper-alloy target having the desiredcopper-alloy composition. The copper-alloy interconnections can bepatterned from a copper-alloy layer with reactive ion etching with achlorine chemistry. The copper-alloy interconnections can also bepatterned by chemical-mechanical polishing.

A goal of the present invention is to provide a high performance, highreliability interconnection for an integrated circuit.

Another goal of the present invention is to provide an interconnectionwhich is not susceptible to electromigration caused hillock and voidformation.

Still another goal of the present invention is to provide a copper-alloyinterconnection which exhibits resistance to oxidation and corrosion.

Still yet another goal of the present invention is to provide a simplemethod of forming a uniform copper-alloy interconnect.

Still other objects and advantages of the present invention will becomeobvious from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a cress-sectional view showing a prior artinterconnect structure.

FIG. 2a is an illustration of a cross-sectional view showing theformation of an interlayer dielectric and metal plugs on a siliconsubstrate.

FIG. 2b is an illustration of a cross-sectional view showing theformation of a barrier layer and a copper-alloy layer on the substrateof FIG. 2a.

FIG. 2c is an illustration of a cross-sectional view showing theformation of individual interconnection lines from the copper-alloylayer and the barrier layer on the substrate of FIG. 2b.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention describes novel, reliable, high performancemetal-alloy interconnections and their method of fabrication. In thefollowing description numerous specific details are set forth such asmaterial types and thicknesses, etc., in order to provide a thoroughunderstanding of the present invention. It will be obvious, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownsemiconductor manufacturing processes and equipment have not beendescribed in detail in order to not unnecessarily obscure the presentinvention.

The present invention describes novel metal-alloy interconnections forintegrated circuits. The preferred embodiment of the present inventionis a copper-alloy interconnection comprising pure copper alloyed with asmall amount (less than five atomic percent) of an additive which has alow residual resistivity and solid solubility in copper. The preferredcomposition of the copper alloy interconnection of the present inventionis pure copper alloyed with between 0.01-1.0 atomic percent of eithervanadium (V), niobium (Nb), or palladium (Pd). The copper alloyinterconnects of the present invention are resistant to oxidation,corrosion, and electromigration. Additionally, the copper alloyinterconnects can have a low resistivity (less than 3 μΩ-cm) and aretherefore ideal for use in modern, high performance integrated circuits.Additionally, the copper alloy interconnects are expected to be somewhatmore resistant to diffusion through ILDs than pure copper interconnects.

Although niobium, vanadium, and palladium are preferred additives in thepresent invention, it is to be appreciated that other elements,compounds, or multiple element alloys may be used. The additive shouldbe able to be alloyed with copper and improve copper's resistance tocorrosion and oxidation without adversely effecting coppers low residualresistivity. The additive should itself have a low residual resistivityso that when alloyed with copper, the resultant alloy will have a lowresistivity. Additionally, the additive should also have low solidsolubility in copper in order to keep the resistivity of thecopper-alloy low. It is to be appreciated that the resistivity of thecopper alloy is dependent upon both the residual resistivity of theadditive and the solid solubility of the additive in copper. Thus, inorder to generate a low resistance conductor, the lower the residualresistivity of the additive, the higher is the acceptable level of solidsolubility.

The percentage of the additive in the alloy varies depending upon thespecific additive used. This is because the residual resistivity ofelements in solid solution varies with additive and is partiallydependent on the additives size relation to copper. The percentage ofthe additive in the alloy is kept small so that the additive canprecipitate to the grain boundaries of the copper matrix. In this waythe copper lattice is not distorted and its resistivity remains low. Iflarge amounts of an additive are used, the additives can distort thelattice and decrease the electron mean free path which translates into ahigher resistance conductor. Additionally, by keeping the percentage ofadditive in the copper-alloy low, the additives are able to diffuse tothe surface of the interconnection and provide a protective layeragainst oxidation and corrosion. It is to be appreciated that thespecific additive and its exact atomic percent used in the alloy can bechosen to optimize the desired electrical resistivity and corrosionresistance of the fabricated interconnection. The composition should bechosen to provide an interconnection with a resistivity lower thanaluminum (˜3 μΩ-cm) in order to generate a high performanceinterconnection.

In fabrication of a copper-alloy interconnection of the presentinvention, a semiconductor substrate 200, such as silicon, in whichactive devices have been formed (not shown) is provided as shown in FIG.2a. Formed on the substrate 200 is an interlayer dielectric (ILD) 202such as SiO₂. ILD 202 is formed to a thickness of between 2,000-20,000 Åby techniques well-known in the art. Metal contacts or plugs 204 extendthrough ILD 200 to active devices formed in substrate 200.

Next, as shown in FIG. 2b, a barrier layer 206 is formed on ILD 202 andon plugs 204. Barrier layer 206 prevents the subsequently formedcopper-alloy layer from diffusing through ILD 202 and poisoning devicesformed in substrate 200. It is to be appreciated that pure copper isknow to readily diffuse through SiO₂ and other ILDs such as polyimidesand parylene. Barrier layer 206 also improves adhesion of thesubsequently formed copper-alloy layer to ILD 202. Barrier layer 206 canbe any of a variety of materials including, but not limited to,conductors such as tantalum, tungsten, and titanium nitride, metaloxides, and dielectrics such as silicon nitride and silicon carbide. Ifa dielectric or metal-oxide is used, then it must be patterned whenlayer 202 is patterned to allow for the plugs 204 to contact the copperalloy layer 208. Barrier layer 206 is formed to a thickness of between50-1000 Å and can be formed with any one of a number of well-knowntechniques including but not limited to chemical vapor deposition (CVD),sputtering, and evaporation.

Next, a copper-alloy layer 208 is deposited onto barrier layer 206.Copper-alloy layer 208 is deposited to a thickness of between 0.1-2.0μ,depending upon the level of metalization. In the preferred embodiment ofthe present invention, copper-alloy layer 208 is formed by sputteringwith well-known techniques and equipment from a specially madecopper-alloy target having the desired alloy composition. A copper-alloytarget can be manufactured by metallurgists, such as Johnson-MattheyInc., of Spokane, Washington, with techniques well-known by thoseskilled in the art. In general terms, a copper-alloy target can bemanufactured by placing copper and the desired amount of an additive,such as niobium, vanadium, and palladium, or a combination thereof, in acrucible under vacuum at a temperature sufficiently high to melt and mixthe copper and the additive together to form a homogeneous alloy. Themolten copper-alloy can then be poured into a mold and machined into astandard target which can be used in well-known sputter depositionsystems. A copper-alloy target having less than five atomic percent ofan additive, with between 0.01-1.0 atomic percent additive preferred, isformed. The copper-alloy target can then used in a well-known sputteringapparatus, such as an Applied Materials Endura sputter machine, to formcopper-alloy layer 208.

Although evaporation or sputtering from a single copper-alloy target isthe preferred method of forming the copper-alloy layer 208, it is to beappreciated that other methods may be employed. For example,copper-alloy layer 208 can be formed by simultaneously sputtering orevaporating from a copper target and an additive target wherein theevaporation or sputtering from the copper target is at a higher ratethan the sputtering from the additive target in order to form acopper-alloy layer with the desired composition. It is also conceivablethat a chemical vapor deposition techniques could be used to form acopper-alloy with the desired composition. It is to be appreciated thatsputtering or evaporating from a single copper-alloy target is preferredbecause the technique presently yields the most chemically uniform alloylayer. Additionally, sputtering from a single copper-alloy target toform as single copper-alloy layer is less complicated and less expensivethat other techniques.

Next, as shown in FIG. 2c, copper-alloy layer 208 and barrier layer 206are patterned into individual interconnection lines 210. In a standardpatterning process, a photoresist layer or masking layer is depositedonto copper-alloy layer 208 and patterned to define the location whereinterconnection lines will be formed. Next, the copper-alloy layer andthe barrier layer are etched away wherever the masking layer has beenremoved. A reactive ion etch using a chemistry comprising chlorine and anoble gas can be used to pattern copper-alloy layer 208 and barrierlayer 206. Although a dry anisotropic etch is preferred, a wet chemicaletch can also be used.

After patterning copper-alloy layer 208 into individual interconnectlines 210, interconnect lines 210 are heat treated. Heat treatingcopper-alloy interconnections 210 diffuses some of the additives in thebulk of the alloy to the surface where they form a protective layer. Inthis way, copper in the interconnection is protected from oxidation andcorrosion. Although not necessarily required, the heat treatment processis preferred in order to improve the corrosion and oxidation resistanceof the copper-alloy interconnections. The protective layer is alsoexpected to reduce copper's diffusion through ILDs such as SiO₂. In thisway a barrier layer may not necessarily be required with thecopper-alloy interconnections of the present invention. Heat treating ofthe interconnection lines can be accomplished by placing substrate 200into a furnace at 200°-700° C. in a nitrogen or ammonia gas ambient forapproximately 10-30 minutes. The heat treatment can be carried out"insitu" with the plasma etch step if desired. In this way theinterconnection lines are not exposed to contaminants in air, prior tothe heat treatment process. Rapid thermal processing can also be used ifdesired.

It is to be appreciated that other methods of patterning thecopper-alloy layer into individual interconnections can be used. Forexample, well-known lift-off techniques can be used. Additionally,interconnect lines can be formed by depositing a copper-alloy intogrooves in a previously patterned ILD and then chemically-mechanicallypolishing back to form individual interconnect lines.

Formation of the copper-alloy interconnect of the present invention isnow complete. It is to be appreciated that additional process steps cannow be used to complete the fabrication of a integrated circuit. Forexample, additional ILDs and copper interconnections can be formed togenerate a multilevel integrated circuit. It is to be appreciated thatalthough the present invention has been described with respect to aninterconnection formed on the first level of metalization, the presentinvention is equally useful for higher levels of metalization (i.e.metal 2 and metal 3, etc.). Passivation layers may also be formed atthis time to hermetically seal the completed integrated circuit fromcontamination.

Additionally, in a similar manner, it is believed that silver alloyscomprising silver and a small amount of an additive such as palladium,vanadium, and niobium, can be used as interconnection metals. Silver hasan extremely low resistivity (˜1.6 μΩ-cm) making it an ideal highperformance interconnection metal. Unfortunately, like copper, puresilver readily oxidizes and corrodes. It is believed, however, thatsilver alloyed with a small amount of additive (0.1-1.0 by atomicpercent) will exhibit resistance to oxidation and corrosion in mannersimilar to copper alloys. A silver alloy layer can be formed bysputtering with a silver alloy target having the desired alloycomposition. The silver alloy layer can then be patterned intoindividual interconnection lines with techniques well-known in the artfor patterning pure silver.

Thus, novel, high performance, high reliability metal-alloyinterconnections have been described.

I claim:
 1. A metal alloy interconnection for an integrated circuitcomprising:silver; and an additive having a low residual resistivity anda low solid solubility in silver wherein said metal alloyinterconnection comprises between 0.01-1.0 atomic percent of saidadditive.
 2. The metal alloy interconnection of claim 1 wherein saidadditive is selected from the group consisting of niobium, palladium,and vanadium.
 3. The metal alloy interconnection of claim 1 wherein saidmetal alloy comprises between 0.01-1.0 atomic percent niobium.
 4. Themetal alloy interconnection of claim 1 wherein said metal alloycomprises between 0.01-1.0 atomic percent palladium.
 5. The metal alloyinterconnection of claim 1 wherein said metal alloy comprises between0.01-1.0 atomic percent vanadium.
 6. A method of forming interconnectionlines for an integrated circuit formed on a semiconductor substratecomprising the steps of:forming an insulating layer above saidsubstrate; forming a silver-alloy layer above said insulating layerwherein said silver-alloy layer comprises:silver; and an additive havinga low residual resistivity and a low solid solubility in silver whereinsaid metal alloy layer comprises between 0.01-1.0 atomic percent of saidadditive; and patterning said silver-alloy layer into saidinterconnection lines.
 7. The method of claim 6 wherein said additive isselected from the group consisting of niobium, palladium, and vanadium.8. The method of claim 6 wherein said silver-alloy layer comprisesbetween 0.01-1.0 atomic percent niobium.
 9. The method of claim 6wherein said silver-alloy layer comprises between 0.01-1.0 atomicpercent palladium.
 10. The method of claim 6 wherein said silver-alloylayer comprises 0.01-1.0 atomic percent vanadium.
 11. The method ofclaim 6 wherein said silver-alloy layer is formed by sputtering from atarget comprising silver and said additive.